There are many benefits to using polar modulation as a means of transmitting information. Polar modulation makes possible the application of the amplitude modulation data signal at the very last stage of a transmitter before an antenna. As a result, all previous amplification stages may be operated in a constant envelope mode and can thus be biased very efficiently. Polar modulation also makes it possible to reduce the current drain quickly as the transmit power is reduced. Polar modulation provides clear talk-time benefits for wireless handset applications.
A common technique used to amplitude modulate a power amplifier includes modulating the power amplifier supply voltage. In the context of a power amplifier based on Gallium Arsenide (GaAs) heterojunction bipolar transistor (HBT) technology, amplitude modulation may be achieved by modulating the collector voltage applied to a HBT device (e.g., transistor) at the output stage of the power amplifier. Typically, the collector voltage may be modulated by introducing a metal oxide semiconductor (MOS) device in series with the power supply (e.g., the battery), which delivers the required current at a controlled voltage to the collector of the HBT device. The voltage drop across the MOS device, however, degrades the optimum efficiency that can be achieved relative to the situation with no MOS device. An additional drawback of the MOS device approach is that the modulation bandwidth that can be supported is limited by the both the capacitive loading associated with the large MOS device, and also by the closed-loop that is typically required to overcome the non-linear characteristic of the MOS device. Accordingly, there is a need for an alternate technique for amplitude modulation.